FIG. 8 is a longitudinal half-sectional view of a spark plug 1. The spark plug 1 includes a tubular metal shell (hereinafter also referred to simply as a metal shell) 10 including portions of different diameters; an insulator 30 having a hollow cylindrical shape that extends through the metal shell 10; and a center electrode 50 that is disposed in a central axial hole in the insulator 30 and that has a tip exposed at a front end (top end in FIG. 8) of the insulator 30. The metal shell 10 includes a ground electrode 20b, which is a bar that is fixed to a front end surface 13 of the metal shell 10 and that is bent inward (toward a central axis O). A spark discharge gap G is formed between the tip of the ground electrode 20b and the tip of the center electrode 50. As illustrated in FIG. 9, the metal shell 10 is manufactured by arranging a bar that serves as a ground electrode member (hereinafter also referred to simply as a bar) 20 so that one end (back end surface) 25 of the bar 20 perpendicularly faces the front end surface 13 the metal shell 10 that serves as a part (single item), and resistance-welding the bar 20 to the metal shell 10. Then, as illustrated in FIG. 10A, the metal shell 10 is subjected to, for example, a process of forming a thread 16 on an outer peripheral surface 15 thereof, and is transferred to a spark plug assembly process. In the spark plug assembly process, the above-described insulator 30 and other components are attached to the metal shell 10 including the bar (see FIGS. 10A and B), and a back end 17 of the metal shell 10 is bent inward and compressed (crimped) toward the front (see FIGS. 10B and 10C). After the assembly, as illustrated in FIG. 10C, the bar 20 is bent inward and serves as the ground electrode 20b that defines a predetermined spark discharge gap.
The bar 20 has conventionally been welded to the front end surface 13 of the metal shell 10 by resistance butt welding as follows. First, as illustrated in FIG. 9, the metal shell (work in process) 10 is secured by being clamped, for example, by one resistance welding electrode (electrode made of copper or a copper alloy, which is not illustrated) of a resistance welding machine (not illustrated). The bar 20 is held by the other resistance welding electrode 210, 220. Then, the back end surface 25 (bottom face in FIG. 9) of the bar 20 is arranged so as to perpendicularly face the front end surface 13 of the metal shell 10 at a predetermined position, and is brought into contact with the front end surface 13. In the contact state, a large electric current is passed between the resistance welding electrodes (the electrode that holds the metal shell 10 and the electrode that holds the bar 20), so that the abutting surfaces of the two members (the front end surface 13 of the metal shell 10 and the back end surface 25 of the bar 20) that are in contact with each other are melted by resistance heating, and are pressed and welded together (see, for example, FIG. 6 of Japanese Unexamined Patent Application Publication No. 2014-135213). In the welding process, the bar 20, which is a ground electrode member, has conventionally been held by clamp-type holding means illustrated in FIG. 9. The holding means includes a pair of electrode lugs 210 and 220 that camp the bar 20 at opposite side surfaces thereof so as to be electrically connected to the bar 20.
The spark plug manufactured by the above-described resistance butt welding process (hereinafter also referred to simply as a welding process) is required to have high dimensional precision and welding (joining) strength to ensure sufficient performance thereof. The bar 20, which is a ground electrode member, may be a thin round bar (columnar member), but is typically a small strip-shaped bar (rectangular bar) that is about 2 to 3 mm wide and about 1 to 1.5 mm thick in cross section and that is about 20 mm long. To precisely and strongly weld such a small bar, it is necessary to precisely control not only the welding conditions, such as the current and welding time, but also the vertical orientation in which the bar 20 is held and the orientation in which the bar 20 is positioned relative to the front end surface 13 of the metal shell 10. In consideration of the above-described requirements and the use of a large amount of current in the welding process, the electrode lugs 210 and 220, which serve as a resistance welding electrode used to hold the bar 20, include clamping portions, that is, contact portions, that are capable of clamping the bar (rectangular bar) 20 at the side surfaces thereof over substantially the entire length in the front-back direction. The contact portions of the electrode lugs, which oppose each other and come into contact with the bar to clamp the bar, have the same front-back length so that the bar can be clamped at the side surfaces thereof in a balanced manner.
Unfortunately, the welding process in which the bar is clamped by the above-described electrode lugs has the following problems. That is, heat is dissipated into the electrode lugs 210 and 220 during resistance heating, and heat concentration and heating efficiency at the abutting surfaces are reduced due to the heat dissipation. This will be described in more detail below. Since the bar 20 is small and short in the front-back direction as described above, electrode lugs that are capable of clamping the bar 20 over substantially the entire length thereof in the front-back direction are used as the electrode lugs 210 and 220 of the resistance welding electrode. Therefore, the electrode lugs are in contact with the bar over a large contact area (contact area between the contact portions and the bar). The electrode (electrode lugs) that holds the bar is typically made of copper (or a copper alloy), which is highly conductive. In addition, since the bar is small, the volume of the electrode lugs is comparatively large. When the bar is subjected to resistance heating by passing a resistance welding current while the bar is clamped by the electrode lugs that are in contact with the bar over a large area, a large portion of the generated heat is transmitted or dissipated into the electrode lugs that clamp the bar, that is, into an electrode of the resistance welding machine. Thus, in the above-described welding process according to the related art, a large amount of heat dissipation occurs during resistance heating. Therefore, the temperature increase at the abutting surfaces to be welded together (the back end surface of the bar 20 and the front end surface 13 of the metal shell 10) is suppressed, and the heating efficiency (heat concentration) is reduced. As a result, in the above-described welding process, the abutting surfaces cannot be efficiently melted and welded together. This is said to lead to a low and insufficient welding strength.
The metal shell of the spark plug is made of a ferrous material, such as low-carbon steel. The bar, which is the ground electrode member, is made of a Ni alloy. Thus, members made of different metals are welded together. Therefore, the welding conditions (current, welding time, and pressing force) need to be set within narrow allowable ranges, and it is difficult to obtain sufficient joining strength. In particular, in the case where the bar has a composite structure including a core made of copper and a peripheral wall made of a Ni alloy in cross section, the welding surface contains the Ni alloy. In such a case, the weldability is low, and the joining strength is easily reduced when the joining surface (cross section) that serves as the welding surface (abutting end surface) is small. Therefore, when the bar having a composite structure is welded, resistance heating needs to be performed efficiently and effectively with small heat dissipation from the viewpoint of preventing breakage of the bar in the subsequent bending step. To reduce heat dissipation to the pair of electrode lugs (electrode), the contact area between the contact portions and the bar may be reduced by reducing the size of the contact portions. However, in such a case, there is a possibility that the bar will be held in an unstable orientation and tilted due to, for example, wear of the electrode lugs. As a result, it becomes difficult to maintain high precision.